1. Field of the Invention
This invention relates to a process for producing a highly water absorptive polymer, and more particularly to a process for producing a highly water absorptive polymer in the form of agglomerated particles having a large particle size.
The agglomerated particles of the highly water absorptive polymer obtained by the process of the present invention are extremely large in diameter, and are excellent in coherence. Therefore, the polymer of the invention can be advantageously used for sanitary goods such as paper diapers and sanitary napkins; for various agents employed in the horticultural and agricultural fields, such as soil conditioners and water-retaining agents; and for materials used in the fields of civil engineering and construction.
2. Background Art
In recent years, highly water absorptive polymers have been widely utilized not only for sanitary goods such as paper diapers and sanitary napkins but also for materials used in industrial fields as well as in horticultural and agricultural fields such as water-holding agents, dropwise condensation preventing materials, freshness preserving materials and dehydrating materials. Under these circumstances, a variety of highly water absorptive polymers have been proposed up to date.
Examples of conventionally known highly water absorptive polymers include a hydrolyzate of a graft copolymer of starch and acrylonitrile, carboxymethyl cellulose, polyacrylic acid (salt), a copolymer of acrylic acid (salt) and vinyl alcohol and polyethylene oxide.
However, none of the conventional highly water absorptive polymers are satisfactory, especially in the size of polymer particles. In particular, polymers obtained by reverse-phase suspension polymerization have various problems. For instance, in the case where an alkali metal salt of acrylic acid or the like is polymerized by means of a water-in-oil type of reverse-phase suspension polymerization using, as a dispersant, a sorbitan fatty acid ester having an HLB of 3 to 6 as described in Japanese Patent Publication No. 54-30710, a nonionic surfactant having an HLB of 6 to 9 as described in Japanese Patent Kokai Publication No. 57-167302, or a surfactant having an HLB of 8 to 12 as described in Japanese Patent Publication No. 60-25045, only a highly water absorptive polymer having a small particle size of approximately 10 to 100 .mu.m can be obtained.
On the other hand, when an acrylic monomer is polymerized by the method disclosed in Japanese Patent Publication No. 63-36321 or 63-36322, using as a dispersant a lipophilic polymer containing a carboxyl group, polymer particles of a size of several hundreds microns can be produced. However, because of a high affinity between the dispersant and the acrylic monomer, polymer particles tend to aggregate excessively in the course of polymerization reaction. Thus, there is a considerable likelihood of formation of a large mass of polymer particles.
Japanese Patent Publication No. 1-17482 and Japanese Patent Kokai Publication No. 57-158210 disclose methods for preparing a highly water absorptive polymer having a large particle diameter, in which an oil-soluble cellulose ester or ether is used as a dispersant. However, these methods have the drawback that the dispersant remaining in the polymer melts upon drying of the polymer. As a result, the polymer particles tend to adhere to one another or to stick to the inner wall of a vessel.
When polymerization is conducted in accordance with the method disclosed in Japanese Patent Kokai Publication No. 62-172006, using as a dispersant a polyglycerine fatty acid ester with an HLB of 2 to 16, a polymer having a large particle size can be obtained. However, the operational conditions for stably producing such a polymer is extremely limited. When the polymerization is carried out under the conditions outside of the limited range, bulk polymerization tends to take place. For this reason, the above method is not advantageous when stable industrial production is taken into consideration.
In the case where polymerization is conducted in the presence of both a dispersant and hydroxyethyl cellulose as disclosed in Japanese Patent Kokai Publications Nos, 56-76419, 62-95307 and 62-95308, a polymer having a large particle size, which is free from the problems of adhesion between the polymer particles and of sticking of the polymer to a vessel, can be obtained. It is however difficult to dissolve the hydroxyethyl cellulose in the monomer, and the use of a special apparatus is required to uniformly dissolve the hydroxyethyl cellulose in the monomer.
As described above, all of the conventional methods for preparing large primary particles of highly water absorptive polymers have various problems.
As an alternative to the above described methods, there has been proposed a method in which primary particles of a highly water absorptive polymer are agglomerated using as a binder water or polyvinyl alcohol.
However, when this method is effected by using water as a binder, the primary particles are agglomerated with an extremely low cohesive force. The agglomerated particles are therefore easily disintegrated upon ordinary transportation or handling. On the other hand, when polyvinyl alcohol is used as a binder, the coherence of agglomerated particles can be improved. However, in this case, it is necessary to use a large amount of polyvinyl alcohol and also to employ a special apparatus, so that the production cost is increased. This method is thus not satisfactory.
Further, Japanese Patent Kokai Publication No. 1-126314 discloses a method in which a monomer component containing a polymerization initiator and a crosslinking agent in larger amounts than those used for preparing a water absorptive polymer (base polymer) is added to and immersed in the water absorptive polymer and is polymerized to make the crosslinking density in the vicinity of the surface of the water absorptive polymer higher than that inside the polymer, thereby improving the water absorption rate of the polymer. However, when this method is applied, the cohesion between the particles of the base polymer is extremely low because the monomer component to be immersed in the polymer contains a polymerization initiator and its amount is larger than that used for preparing the base polymer.
It is therefore a primary object of the present invention to provide a process for producing agglomerated particles of a highly water absorptive polymer, which are very large in size and excellent in cohesion, stably and at low cost.